(CNN) -- After 10 days of extensive searching and following up on hundreds of tips, the case of a missing Oregon boy has been classified as a possible crime, authorities announced Sunday.

"With the search-and-rescue mission completed at this time, we are moving operations toward the criminal investigative end of the spectrum," Capt. Monte Reiser of the Multnomah County Sheriff's Office said at a briefing. "While we still are keeping all options open, it is a natural progression to move the overall focus to a criminal case."

Until this point, law enforcement authorities had called the investigation into the disappearance of 7-year-old Kyron Horman a missing endangered child case and indicated there was no evidence of criminal activity.

Kyron was last seen the morning of June 4 walking down the hallway of Skyline Elementary School in Portland as he headed to his second-grade classroom, his stepmother has told investigators, according to police.

Multnomah County Sheriff Dan Staton would not disclose Sunday why the case has now been labeled a criminal probe.

"There are still a number of details that we cannot get into with regards to what we are doing and how we are doing it," he said.

Staton also announced a $25,000 reward in return for specific information that leads to Kyron's whereabouts.

He again asked the public for help in finding the child.

"We have said many times over the past 10 days that we need your tips," Staton said. "It is oftentimes the smallest detail -- something that may seem inconsequential to you -- that is the piece of the puzzle we need to crack this open."

On Friday, the family of Kyron made a tearful plea for his safe return.

"Please help us bring Kyron home," said Kaine Horman, the boy's father. "We as the family know how difficult and stressful this is but your memories and statements can help us find Kyron."